1. Field of the Invention
The present invention relates to a distance measuring device for use in a camera and, more particularly, to a device which can automatically determine the distance from the camera to a subject to be photographed by the application of triangulation technique, and can automatically focus the camera lens in accordance with the determination.
2. Description of the Prior Art
Many cameras employing a distance measuring device have been proposed, and one of which is disclosed in Tokkosho (Japanese Patent Publication) 54-39731 published Nov. 29, 1979. The basic principle of the distance measuring device disclosed in this prior art involves directing onto the subject a beam of light emitted from a light source positioned in predetermined positional relationship with respect to the camera; receiving, at a location kept apart from the light source by a predetermined base length, a beam reflected back to the camera from the subject; and detecting the position on the receiving device of the beam reflected from the subject, in order to determine the distance to the subject.
A light receiving device which is disclosed in Tokkosho 54-39731 is shown in FIG. 1, and includes a pair of rectangular photosensitive elements 2 and 4 positioned on opposite sides of a predetermined base line 6 of the triangulation theory with their longer side edges being parallel to the base line 6, and a mask 8 having an opening 8a formed therein. The mask 8 is positioned in front of the photosensitive elements 2 and 4 so that the area available for receiving the reflected light beam by the photosensitive elements 2 and 4 is restricted to the area defined by the opening 8a. As shown in FIG. 1, the lower side edge of the opening 8a extends in a direction which is parallel to the base line 6 but the upper side edge is slanted such that the distance between the upper edge of the opening 8a and the base line 6 becomes greater towards right-hand direction. The light receiving device shown is provided in association with an optical arrangement (not shown) including a cylindrical lens for forming an elongated line image 10 which is perpendicular to the base line 6, said image 10 being formed on the photosensitive elements 2 and 4, and said image 10 being caused by the light reflected from the subject. The line image 10 is formed so long that it may extend across the opening 8a. Thus, no matter where the image is located, it may extend a section of the opening 8a from upper edge to the lower edge. The position of the line image 10 along the base line 6 has relationship with respect to the distance to the subject from which the reflected beam is directed. More particularly, when the distance to the subject is comparatively short, the line image 10 will be located at the left-hand end of the opening 8a, and as the distance to the subject increases, the line images 10 shifts rightwardly towards the opposite end of the opening 8a. When the distance to the subject is very far, the line image 10 will be located at the right-hand end of the opening 8a. When the photosensitive elements 2 and 4 receive the line image 10, the output signal of the photosensitive element 2 relates to both the position along the base line 6 and the light intensity of the line image 10, while the output signal of the photosensitive element 4 relates to only the light intensity of the line image 10. Thus, the difference in output signals between the photosensitive elements 2 and 4 relates to only the position of the line image 10 along the base line 6, and this difference provides the information of the distance to the subject.
When it is desired to incorporate the above described device in a compact size camera while maintaining a predetermined distance measuring accuracy, the area of the photosensitive elements 2 and 4, particularly the area available for receiving light therein, will be reduced, and the width of the line image 10 must be made very thin. Although this can be accomplished, e.g., by the employment of a slit in front of the light source for emitting a narrow strip-like beam or by the use of optical arrangement in the receiving device capable of forming a very thin line image 10 on the photosensitive elements 2 and 4, such a slit or arrangement absolutely reduces the light amount received by the photosensitive elements 2 and 4, and as an inevitable consequence thereof the, signal produced from the elements 2 and 4 becomes very weak, making it difficult to detect the distance to the subject, particularly when the subject is located far from the camera.
This defect can be solved, e.g. by the employment of a powerful light source actuated by a powerful battery, but the result is such that the camera will become considerably large in size and heavy in weight. Therefore, from this point of view, it is difficult for the distance measuring device of the above described prior art to be made compact in said while maintaining a predetermined distance measuring accuracy. Furthermore, since the mask 8 intercepts the end portions of the line image 10 by the upper and lower edges of the opening 8a, the reflected light beam received by the elements 2 and 4 is less than the light which has passed through the cylindrical lens positioned in front of the elements 2 and 4. This also weakens the signal produced from the elements 2 and 4.
Furthermore, when the subject has a different reflectance between portions, e.g., between upper and lower portions, the intensity of the light beam received by the element 2 and that by the element 4 will be affected by the different reflectance, and therefore, the signals obtained from the elements 2 and 4 will not represent the correct distance to the subject.